Preheat the oven to 375°F (190°C).
Place the diced potatoes in a large pot and cover with cold water. Bring to a boil over high heat, then reduce to medium and cook until potatoes are tender, about 10-12 minutes. Drain and set aside.
In a medium saucepan, melt the butter over medium heat. Whisk in the flour and cook for 1-2 minutes until lightly golden and bubbly, stirring constantly.
Gradually whisk in the milk, continuing to whisk until the mixture thickens and becomes smooth, about 3-5 minutes.
Remove the saucepan from heat and stir in the sour cream, garlic powder, smoked paprika, salt, and black pepper until well combined.
In a large mixing bowl, combine the cooked potatoes, shredded chicken, chopped onion, half of the cheddar cheese, half of the mozzarella cheese, and the sauce mixture. Gently fold until everything is evenly coated.
Transfer the mixture to a greased 9×13 inch baking dish and spread evenly.
Sprinkle the remaining cheddar and mozzarella cheeses evenly over the top.
Bake uncovered for 30-35 minutes, or until the cheese is melted, bubbly, and golden brown on top.
Remove from the oven and let cool for 5 minutes before garnishing with chopped fresh parsley.
Serve warm and enjoy!